docs(ref): Specify frontmatter

src/SUMMARY.md

@@ -8,6 +8,7 @@
     - [Input format](input-format.md)
     - [Keywords](keywords.md)
     - [Identifiers](identifiers.md)
+    - [Frontmatter](frontmatter.md)
     - [Comments](comments.md)
     - [Whitespace](whitespace.md)
     - [Tokens](tokens.md)

src/frontmatter.md

@@ -0,0 +1,42 @@
+r[frontmatter]
+# Frontmatter
+
+r[frontmatter.syntax]
+```grammar,lexer
+FRONTMATTER ->
+      FRONTMATTER_FENCE INFOSTRING? LF
+      (FRONTMATTER_LINE LF )*
+      FRONTMATTER_FENCE[^matched-fence] LF
+
+FRONTMATTER_FENCE -> `---` `-`*
+
+INFOSTRING -> (XID_Start | `_`) ( XID_Continue | `-` | `.` )*
+
+FRONTMATTER_LINE -> (~INVALID_FRONTMATTER_LINE_START (~INVALID_FRONTMATTER_LINE_CONTINUE)*)?
+
+INVALID_FRONTMATTER_LINE_START -> (FRONTMATTER_FENCE[^escaped-fence] | LF)
+
+INVALID_FRONTMATTER_LINE_CONTINUE -> LF
+```
+
+[^matched-fence]: The closing fence must have the same number of `-` as the opening fence
+[^escaped-fence]: A `FRONTMATTER_FENCE` at the beginning of a `FRONTMATTER_LINE` is only invalid if it has the same or more `-` as the `FRONTMATTER_FENCE`
+
+Frontmatter is an optional section for content intended for external tools without requiring these tools to have full knowledge of the Rust grammar.
+
+r[frontmatter.document]
+Frontmatter may only be preceded by a [shebang] and whitespace.
+
+r[frontmatter.fence]
+The delimiters are referred to as a "fence."
+The opening and closing fences must be at the start of a line.
+They must be a matching pair of three or more hyphens (`-`).
+
+r[frontmatter.infostring]
+Following the opening fence may be an infostring for identifying the intention of the contained content.
+An infostring may be preceded by non-newline whitespace.
+
+r[frontmatter.body]
+The body of the frontmatter may contain any content except for a line starting with as many or more hyphens (`-`) than in the fences.
+
+[shebang]: input-format.md#shebang-removal